1. Field of the Invention
The present invention relates, in general, to a magneto-optical recording medium for use with short wavelengths and, more particularly, to a magneto-optical recording medium with high data storage density, high data transfer rates and long data archival capability.
2. Description of the Prior Art
The variety of information has explosively increased in the present information-intensive society. Such an information explosion requires a recording media to be higher in data storage density and data transfer rate and to be faster in operation.
Currently, the practical or commercial techniques for recording data are substantially based on magnetic recording technology. In general, data are stored on magnetic media, such as video tapes, audio tapes, floppy disks and the like, on which information is recorded depending on the direction of magnetization of magnetic substances in the magnetic recording media.
While the magnetic recording technology is commercially successful and advantageous, a recording technique known generically as optical recording has been and continues to be considered a very promising alternative for the storage of data, optical recording media has a higher capacity which is demanded in response to the enormous amount of information resulting from the society""s development.
A magneto-optical recording medium, in contrast with the magnetic medium, comprises a recording layer magnetizable in perpendicular to the plane of the layer itself. In addition, the coercive force (Hc) of magneto-optical recording medium, a strength capable of being maintained in the magnetized state, is about 5 to 10 times as high as that of a magnetic medium. Accordingly, it is very difficult to change a previous direction of magnetization with an external magnetic field. The recording of information on a magneto-optical recording layer is effected by first focusing a modulated laser beam on a surface of the layer within 1 xcexcm in diameter, the laser beam power being sufficient to heat the layer locally, for example, to the Curie point temperature of the layer. In this state, a direction of magnetization can be changed with an external magnetic field, so as to record information on the layer according to the direction.
When information is recorded by this method, the recorded unit of information comes to be reduced into 1 xcexcm or less. Accordingly, the recording density of magneto-optical recording medium is 10 to 1,000 times greater than that of a conventional magnetic recording medium. In addition, the magneto-optical medium employs a non-contact reading method, so that magneto-optical recording potentially has significant advantages over magnetic recording, including easier data preservation and longer data archival capability.
In the last few years, personal computers have begun to appear in the market; with a multimedia system as the most widely adopted structure, wherein moving picture information is processed. Additionally recording systems which demand a recording medium a quite-high data storage density, for example, optical filing systems, digital-video disk recording systems and so on have become widespread. To meet the demand, magneto-optical recording media for use with short wavelengths has been employed.
The first generation magneto-optical recording disk typically has a data storage capacity of 128 MB or 650 MB in a diameter of 3.5 inches or 5.25 inches for each. The 3.5 inch magneto-optical recording disk is usually used in a personal computer whereas the 5.25 inch is introduced into an optical filing system. Since these magneto-optical recording disks comprising a recording layer of TbFeCo film take advantage of a laser beam with a wavelength of 830 nm, they are superior in perpendicular magnetization and high in magneto-optical effect (Kerr rotation angle, "THgr"k), showing a carrier to noise (C/N) ratio of 45 dB or greater.
However, much larger capacity is required, in order to process more information which is now needed by many electromagnetic systems. For example, a capacity of 6 GB or more is required for a recording medium which is used to process moving pictures of high definition television quality in an optical filing system for a super computer or a digital-video disk recording system. In optical recording art, it is considered that the use of a laser beam having shorter wavelength brings about higher data storage density. For example, a blue laser beam with a wavelength of 400 nm, compared to a wavelength of 830 nm, makes a laser beam-focused spot reduced to one fourth in area, which results in an improvement of four times higher recording density. Accordingly, magneto-optical recording media for use with a short wavelength are strongly demanded.
Referring to FIG. 1, there is shown a conventional magneto-optical recording medium having a multilayer structure wherein substrate 1, dielectric layer 2, recording layer 3, protective layer 4 and reflective layer 5 are, in sequence, laminated. As shown in this figure, the recording layer of the conventional magneto-optical recording medium is a single layer made of TbFeCo, an alloy of rare-earth metal and transient metal. At a laser wavelength of 830 nm, this TbFeCo layer is favorably magnetized perpendicular to the plane of itself and thus, information is recorded well therein. In addition, it shows good characteristics on readout of information because of its large Kerr rotation angle at this laser wavelength.
When used at a short wavelength region (below 532 nm), the monolayer consisting exclusively of TbFeCo, however, represents a low Kerr rotation angle which is only 60% of that at 830 nm. Under the condition, the conventional magneto-optical recording medium comprising the recording monolayer exhibits a C/N ratio of 45 dB or less and thus has difficulty in readout of information.
As explained above, the conventional magneto-optical recording medium which has a recording layer of TbFeCo just above a dielectric layer on a substrate has a large Kerr rotation angle enough to present reliable reproducing characteristics in addition to being good in perpendicular magnetization and recording characteristics, at a laser wavelength of 830 nm. In contrast, at a short wavelength range of not more than 532 nm, it suffers from a problem on readout of information, for the Kerr rotation angle is 60% as high as that at 830 nm, lowering the C/N ratio to 45 dB or less.
The present inventor has intensively researched and studied these problems and found that a reproducing/recording bilayer structure inserted between the dielectric layer and the protective layer enables the magneto-optical recording medium to display superior magneto-optical properties even at a short wavelength range of not more than 532 nm.
Accordingly, it is an object of the present invention to overcome the above problems encountered in prior art and to provide a magneto-optical recording medium which is superior in magnetic and magneto-optical properties at a short wavelength range.
It is another object of the present invention to provide a magneto-optical recording medium for short wavelength with high data storage density, high data transfer rate and long data archival capability.
In accordance with the present invention, the above objects could be accomplished by providing a magneto-optical recording medium for use with short wavelength in which a dielectric layer, a protective layer and a reflective layer are, in sequence, formed on a substrate, the magneto-optical recording medium comprising a reproducing/recording bilayer structure between said dielectric layer and the protective layer, said bilayer being made of (Ndx(TbFeCoCr)100xe2x88x92x)/(TbFeCoCr).